THE COMPOSITION OF STARSGE
TT
Y
we should be able to do t he sa me
thing for the Sun. But people would
say we have gone mad to dream of
such a thing.”
Nevertheless, they turned their
attention to the spectrum of the Sun
and found that many of the dark lines
found by Fraunhofer were in t he sa me
part of the spectrum – at precisely the
same wavelengths – as the bright lines
produced by various elements when
heated in the lab.
The implication was that these
elements are present in the outer layer
of the Sun. It was thought that, as light
from the hot interior passes through
this region, these elements remove
light from the spectrum at specific
wavelengths instead of adding bright
lines to it. Kirchhoff, in particular,
developed this understanding of
what was going on.
Nobody at t hat time k new precisely
how the lines were produced. But even
without that understanding, in the
1860s it beca me possible to f ind out
what the Sun was made of – and, using
the same technique, what the stars
were made of.
Referring back to their riverside
conversation, Kirchhoff is said to
have told his colleague, “Bunsen,
I have gone mad.” Bunsen replied,
“So have I, Kirchhoff!”Stellar discovery
In the last decades of the 19th century,
ast ronomers were able to identif y t he
presence of many elements found on
Earth in the spectrum of the Sun and,
with less detail, the stars. The natural
assumption t hey made was t hat t he
overall composition of the Sun was
rather like the overall composition of
Earth. But this turned out to be wrong.
Stars are much simpler than that and
we now k now t hat t hey (t he Sun
included) are mostly composed of
hyd rogen a nd helium wit h just t races
of the other elements.
But at the beginning of the 1860s,
nobody even knew there was such a
thing as helium. Its discovery marked
the coming of age of solar – and stellar- spectroscopy.
The leading light in the discovery of
helium was the British astronomer 5
The big questions that scientists are
still hunting for answers toWHAT WE STILL DON’T
KNOW ABOUT STARS
What conditions made star formation possible?
Initially, the Universe was too energetic for stars to form. But as the Universe
expanded and cooled, it became possible for gravity to form clumps of gas.
There’s a suggestion from the European Planck satellite that conditions
made it possible for stars to form within 500,000 years of the Big Bang,
but there is uncertainty about these early years. Space telescopes and
cosmic microwave background detectors will help us discover more about
the early Universe.The mechanics of supernovas
Although there are theories on how supernovas work, there’s not enough
evidence to be sure that these theories are correct. As an example, neutron
stars often leave a supernova explosion at high speed, but no-one knows
why the explosion should favour one direction only. Some of the most
useful supernova observations come from X-ray and gamma ray space
telescopes such as Chandra and NuSTAR, which constantly add data that
may help us understand these massive stellar explosions.Are there Population III stars?
Stars are classified either as Population I (metal-rich) or Population II
(metal-poor). The older Population II stars contain fewer heavy elements,
because the young Population I stars gain heavy elements from supernovas.
But cosmological models suggest that there should also be huge, ancient
Population III stars, made almost entirely from hydrogen and helium, and
created soon after the Big Bang. These are yet to be detected, but the James
Webb Space Telescope, set to launch in 2021, could change that.An ar tist ’s impression of
a supernova explosion